This invention generally relates to the field of radio frequency amplifiers, more particularly, to radio frequency amplifiers that linearly amplify radio frequency (RF) carrier signals.
Radio Frequency (RF) power amplifiers are used for transmission of low-frequency baseband signals that are modulated over radio frequency (RF) carrier signals. In communication systems that use multiple narrowband carriers, broadband CDMA channels, or where linear modulation formats are used, highly linear power amplifiers are needed to preserve the baseband signal""s phase and amplitude. High linearity also prevents carrier signals from intermodulating with each other and avoids broadening of the spectra of the baseband signals, which causes interference in other channels.
Many methods are known that provide high efficiency for low-frequency linear amplifiers, such as Class D PWM switching, Sigma-Delta switching, etc. At low frequencies, feedback loops are commonly used for linearizing a baseband signal that may be amplified with high efficiency. In order to be communicated over RF channels, the baseband signal must be up-converted to an RF carrier signal, which is transmitted with high power. As a result, the linearity of the base band signal modulated over the high power carrier signal should be maintained.
Conventional methods for linearization in high-frequency power amplifiers include linear amplification with non-linear components (LINC), combined analog locked loop universal modulator (CALLUM), adaptive baseband predistortion, and Feedforward linearization. These and other linearization methods are described by Mats Johansson of Department of Applied Electronics-Lund Institute of Technology-in Linearization of Wideband RF power Amplifiers Using Modulation Feedback, which is hereby incorporated by reference. At high frequency, however, the application of a feedback loop complicates linearization. This is mainly due to the fact that phase shift around the feedback loop increases with frequency. It is also difficult to achieve high efficiency with high power amplifiers at high frequency.
In order to overcome complications associated with linearization at high frequency, one conventional approach linearizes a baseband signal before up-converting it. This approach applies a linearized baseband signal at a low-power stage to an up-converting mixer, which mixes the baseband signal with a local oscillator signal to produce the carrier signal. Then, the up-converted baseband signal is amplified by a high-power stage at high frequency. FIGS. 1(a) and 1(b) show the spectra of a signal at baseband and carrier frequency, respectively. However, due to its switching characteristics, a conventional up-converting mixer often reintroduces non-linearities to the generated carrier signal. Furthermore, the high-power stage usually includes one or more RF power transistors that amplify the up-converted baseband signal. RF power transistors, however, have lower gain, lower efficiency, and lower power than power transistors that amplify baseband signals. The RF power transistors are also more susceptible to reactive coupling at carrier frequency than at baseband. It is also easier to linearize and amplify at baseband. Consequently, RF power transistors cause amplitude-to-phase distortion in addition to amplitude distortion.
Therefore, there exists a need for an efficient power amplifier that provides a high power linear carrier signal at high frequencies.
Briefly, the present invention that addresses this need is exemplified in a linear power amplifier that mixes a local oscillator signal with a linear baseband signal using optically activated switches that are activated according to a switching sequence.
The linear amplifier includes an amplifier stage that amplifies the baseband signal, which has linear In-phase (I) and Quadrature-phase (Q) components. A local oscillator generates a local oscillator signal with a specified carrier frequency to be mixed with the linear baseband signal. An up-converting mixer includes a plurality of optically activated switches that are switched according to the switching sequence to mix the local oscillator signal with the linear baseband signal, without re-introducing non-linearities. Preferably, a filter stage isolates the amplifier stage from the up-converting mixer. An optical controller controls the switching sequence in a way that a constant impedance is presented over each carrier frequency cycle. According to the switching sequence, the optical controller activates the optically activated switches during corresponding fractions of the carrier frequency cycles. A fraction of a carrier frequency cycle during which an optically activated switch is closed is defined as duty cycle of the switch and is expressed in terms of a percentage. Therefore, the periods of the optically activated switches are percentages of time during the carrier frequency cycle that the switches are closed. In order to present the constant impedance, the switching sequence is selected such that none of the optically activated switches are closed concurrently. Therefore, the duty cycles of the optically activated switches are non-overlapping.
According to some of the more detailed features of the invention, the power amplifier stage includes a first power amplifier that produces a pair of complementary I components and a second power amplifier that produces a pair of complementary Q components. Using the optically activated switches, the up-converting mixer mixes the local oscillator signal with the pairs of complementary I and Q components.
In one exemplary embodiment, the plurality of the optically activated switches include first, second, third, and fourth optically activated switches that are respectively coupled to first and second complementary I components and first and second complementary Q components. Over repetitive carrier frequency cycles, each optically activated switch is activated during its corresponding fraction of the carrier frequency cycle. Preferably, for each optically activated switch, the duty cycle is equal to 25% of the carrier frequency cycle. Under this arrangement, the switching sequence sequentially closes the first optically activated switch, followed by the third optically activated switch, followed by the second optically activated switch, and finally the fourth optically activated switch.
In yet another more detailed feature of the invention, the optical controller includes one or more pulsed light sources, such as laser sources, for generating light pulses that control the optically activated switches according to the switching sequence. In one exemplary embodiment, the optical controller that controls the switching sequence has a single pulsed light source that is coupled to optic links having delay lines corresponding to fractions of the time period T of the frequency cycle, preferably, differing multiples of 0.25T. In another exemplary embodiment, the optical controller achieves the desired switching sequence by interleaving multiple pulsed light signals that are generated by corresponding phase-shifted local oscillator signals, which are applied to multiple pulsed light sources.
Other features and advantages of the present invention will become apparent from the following description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.